Also, what size tank is this?
Dropsy is not curable. Sometimes, if caught soon enough, it is possible to treat the symptoms, but it is always fatal in the end. One thing encouraging is that it is not transmitted like most other fish illnesses. The only way for one fish to give it to another is if a healthy fish eats the internal organs of a sick fish that has died. With this in mind, are we sure this is dropsy that you're dealing with and not an internal parasite?
I would look most for the protruding scales that Blue mentioned. The scales will look like they are "standing up" or "sticking out" individually from each other.
Something else I'm wondering about... dropsy tends to hit specific species of fish, and loaches, catfishes, and neons are not on that list.
What kind of catfish are we speaking of? How long has your tank been set up, and do you have water params for us to review?
Also, what are you feeding the fish and how often? What kind of filter are you using, and how often do you do water exchanges? How much at a time? How often do you vacuum the gravel?
The more information you can give, the faster we can help.
Very sorry to hear about your loss. I'll do whatever I can to help make sure it doesn't continue or happen again with new fish.
At this moment, I would say best not to add more fish until we know for sure what the problem is. If you were closer I'd be asking you to bring me a body for autopsy. There are tell tale signs of most illnesses, and an examination of a freshly decesased fish always makes it easier to determine.
When the fish died, did you notice any other symptoms? Changes in habits? Red gills? The more you can provide for info, the more we can help to get an accurate diagnosis.